The leading 3-D optical memory technology is holographic data storage. FIG. 1 illustrates a typical holographic memory apparatus for recording data in a 3-D holographic memory, of the type disclosed generally by VanHeerden in "Theory of Optical Information Storage in Solids," Appl. Opt. 2, 393-400 (1963), and von der Linde et al., "Photorefractive effects for Reversible Holographic Storage of Information," Appl. Phys. 8, 85-100 (1975). As illustrated in FIG. 1, the information to be stored is represented in a binary format on a two-dimensional spatial light modulator (SLM). The SLM may be transmissive, as illustrated in FIG. 1, or reflective. To achieve the high coherence required of the image beam and the reference beam, a common laser beam is split into two paths. Light from one path illuminates the SLM while light in the other path becomes a reference beam. The reference beam path is directed at an appropriate angle to achieve a high diffraction efficiency in the volume holographic material. The use of image and reference beam is manner described by Kogelnik in "Coupled Wave Theory for Thick Hologram ratings," Bell System Technical Journal 48, 2909-2947 (1969). The two beams are directed in such a way that they intersect within the recording medium. Because the beams are coherent with respect to each other, an interference pattern is created. The interference pattern is recorded in the holographic medium.
The interference pattern has a unique spatial frequency content that is determined by the properties of the SLM, the SLM pattern, and the angle between the reference and signal paths. By changing the reference beam angle, another holographic pattern can be stored in the same physical location without destroying the data from previously recorded patterns. This process is referred to as angle multiplexing, and is possible as the spatial frequency properties of successive recordings are substantially different (due to the different reference beam angles) so that each SLM pattern can be individually recovered. The SLM pattern may be recovered by illuminating the recording medium with a reference beam whose angle corresponds to the reference beam angle used during recording. The optical memory output, which is imaged onto an electro-optical readout device (e.g. a solid state camera) is a recreation of the SLM data presented to the system during recording. Additional data can be recorded if the recording medium is mechanically rotated around an axis perpendicular to its surface, in a process known as peristrophic multiplexing. Peristrophic multiplexing is described by Curtis et al., "Method for Holographic Storage Using Peristrophic Multiplexing," Opt. Lett. 19, 993-995 (1994), and in U.S. Pat. No. 5,483,365 to Curtis et al.
Holographic memories technology has been under theoretical development for some time, as described in "Experimental Holographic Read-Write Memory Using 3-D Storage" by d'Aurie et al., Appl. Opt. 13, 808-818 (1974). Holographic memories have theoretical capacities on the order of 10.sup.12 bits/cm.sup.3 and parallel, page-oriented readout, which provides a convenient interface for storage of images and other page-oriented data. Access times in systems using parallel writing and reading are potentially much faster than access times achieved in systems using bit-oriented 3-D storage, as noted by Strickler et al. in "Three Dimensional Optical data storage in refractive media by two-photon point excitation," Opt. Lett 16, 1780-1782 (1991).
However, in practice, maximum theoretical capacities and response speeds have not been realized and holographic memories have not achieved widespread commercial acceptance. Through their research, the inventors have noted that systems of the type shown in FIG. 1 require the integration of a number of optical components with separate optomechanical mountings, alignment requirements, and air-glass interfaces. The implementation of angle multiplexing is generally accomplished using an electromechanical mechanism such as a scanning mirror, and peristrophic multiplexing requires a means by which the recording medium can be rotated. The inventors have found that numerous design features of these systems, including the noted optical interfacing techniques and mechanical scanning components, limit the achievable bandwidth, access time, and storage density of these systems.
U.S. Pat. No. 5,671,073 to Psaltis et al. discloses a holographic memory storage device of this type, including a mechanical beam steering assembly for peristrophic and angular multiplexing of an optical recording medium. U.S. Pat. No. 5,436,867 to Mok shows a holographic random access memory with an optical mechanism for angular multiplexing, but the system shown in Mok does not provide an integrated non-mechanical system for peristrophic multiplexing.
U.S. Pat. Nos. 4,860,253 to Owechko et al., 5,126,869 to Lipchak et al., 5,132,811 to Iwaki et al., 5,519,651 to Redfield., 5,661,577 to Jenkins et al., 5,671,073 to Psaltis et al., 5,483,365 to Pu et al., 5,550,779 to Burr et al., and 5,638,193 to Trisnadi et al. show other holographic memory storage devices and methods from the prior art. Systems falling into the general categories shown in these patents share the performance limitations described above.
The concept of a liquid crystal grating has been suggested in the scientific community (see, for example, W. P. Parker, "Commercial Applications of Diffractive Switched Optical Elements (SOE's)," Proceedings of the SPIE 2689, 195-209 (1996)) but such devices are not widely available.
Therefore, the inventors have determined that there is a need for improved systems and methods which can be applied to holographic memory storage interfaces.